Various transportation systems have been developed in which a passive transport or conveying unit is propelled along a path or track by frictional engagement of the unit by a frictional drive assembly. The drive assembly most commonly used is a series of wheels which sequentially engage the transport unit and drive it along the track or path. In some systems the drive assembly simply engages a portion or traction surface on the body of the transport unit. Other transport systems employ a traction structure or drive shoe which is carried by the transport unit and engaged by drive wheels, gears or belts.
At least two common broad types of transport systems employing device shoe assemblies are well known, namely, general purpose transportation systems in which passive units are driven over the length of the system and haul rope-based aerial tramway systems in which detached units are moved over short distances. Typical of a general purpose transportation system employing transport units with drive shoes and frictional driving assemblies is the transportation system disclosed in my U.S. Pat. No. 4,671,186. In my patent, drive wheels or gears engage a drive shoe mounted on the side of the cabin of a transport unit. The drive shoes are rigid or substantially inflexible, but they can be resiliently mounted and biased toward the drive wheels to smooth propulsion of the transport units.
Other prior art general purpose transport systems in which drive wheels, gears or belts engage and frictionally propel transport units are disclosed in U.S. Pat. Nos. 4,368,037; 4,152,992; 4,078,499; 3,880,088; 3,871,303; 3,759,188; 3,735,710; 3,039,402; 2,905,101; 496,188 and 482,469; and French Patent Nos. 1,354,297 and 1,300,029. In each of these systems the assembly or portion of the transport unit engaged by the frictional drive means is a relatively inflexible or rigid traction surface.
There are two general categories of aerial tramway haul rope grip assemblies in widespread use, namely, those with detachable grip assemblies and those with permanently affixed grip assemblies. As used henceforth, the expression "aerial tramway" shall be understood to include any haul rope-based conveying system of the type transporting a plurality of passengers or cargo carrier units (e.g., chairs, gondolas, cabins, platforms) secured to a haul rope to enable those units to be conveyed along a path. Detachable grip assemblies are regularly removed from the tramway haul rope, usually to permit the transport unit to be slowed down below the haul rope speed for ease, comfort and safety of loading and unloading at the tramway terminals or stations.
Typical of an aerial tramway system in which detached passive transport units are conveyed along a track or rail is the tramway of my U.S. Pat. No. 4,744,306. Such aerial tramways advantageously employ detachable grip assemblies which also carry a drive shoe or traction member that is engaged by drive wheels. My U.S. Pat. Nos. 4,658,733 and 4,860,664 disclose combined detachable grip and drive shoe assemblies of the type which may be employed in aerial tramways.
The detachable grip assembly of my U.S. Pat. No. 4,860,664 is illustrated in FIG. 1 of the present drawing. In this tramway system the detached carrier unit is propelled around the tramway terminal on rails by drive tires on wheels at a speed much slower than the haul rope. The drive wheels provide a means for accelerating and decelerating the carrier unit at the terminal for loading and unloading of passengers.
When disengaged from the haul rope, this detachable grip assembly, generally designated 30, and their carrier units (not shown), are normally supported at the tramway terminals by rolling elements, such as rollers 32, which travel along a path defined by guide rails 34, shown in FIG. 2. To propel grip assembly 30 along guide rails 34, an upwardly facing traction drive shoe assembly 38, mounted to grip assembly 30 and having a relatively rigid traction surface 40, engages with a plurality of stationary, sequenced, rotatable wheels 36 positioned along guide rails 34. Rigid traction shoes are old in the art and are commonly employed for the above-mentioned purpose. Drive shoe assembly 30 is urged around the terminal at a speed proportional to the angular velocity of drive wheels 36. Thus, in order to propel the carrier unit around the terminal, via drive shoe assembly 38, the coefficient of friction between rotatable wheels 36 and traction surface 40 must be sufficient enough to enable proper movement along the provided path.
One constantly reoccurring problem in aerial tramway transport systems employed in cold and snowy environments is that snow and ice can build up on the traction surfaces used to propel the transport units. For wheel-driven systems that engage horizontally oriented, upwardly facing drive shoes, such as shoe 38, the problem occurs when ice builds up on drive shoe 38. Ice build-up can be so severe that the units cannot be moved along even relatively level rails 34.
For aerial tramways the approach most frequently taken to reduce this problem is to brush or clean drive shoe 38. For example, in the aerial tramway system of U.S. Pat. No. 4,563,955 snow scrapers and powered rotating brushes are mounted above and at an angle to the drive shoe to sweep snow and debris off the shoe or friction plate. While effective for snow, such systems are less reliable and relatively ineffective in removing ice. Thus, even when brushes are used, weather can ice-up and close down tramway systems, which, of course, is highly undesirable in applications such as are common in the skiing industry.
Another problem associated with aerial tramway transportation systems occurs when one or more of drive wheels 36 deflates. A deflated tire is analogous to an excessive accumulation of ice on traction surface 40. That is, a deflated tire lacks sufficient rigidity to enable wheel 36 to propel drive shoe assembly 38 forward. The carrier unit may become delayed or stalled in these areas of deflation. Since an aerial tramway having detachable grip assemblies may employ more than 200 inflatable tires, even a small percentage of deflations can be significant, especially if two or more consecutive tires are deflated.
A simple solution to the deflation problem would seemingly be to replace inflatable wheels 36 with solid or relatively rigid tires. Solid rubber tires, for example, are relatively maintenance free, more durable and less costly. Unfortunately, because traction surface 40 of drive shoe assembly 38 is relatively rigid, the resilient inflatable tires 36 are highly desirable to absorb excess energy created when engagement occurs. Moreover, the cabins or chairs often are swinging in a direction lateral to the haul rope when they enter the terminals, and resilient inflatable tires cooperate with the rigid drive shoes to damp lateral sway or swinging. Accordingly, drive wheels 36 are provided as inflatable tires because of their resilient properties.
Lastly, drive wheels 36 operating near the entrance or exit of the terminal are driven at differing angular velocities to accelerate or decelerate the carrier units. That is, consecutive wheels 36 operating at different speeds gradually accelerate and decelerate the carrier unit as rigid traction surface 40 engages wheels 36. During acceleration and deceleration it is further desirable to maintain traction surface 40 in constant engagement with at least one drive wheel 36, to potentially reduce stalling between wheels 36. Thus, the length of traction surface 40 preferably spans the distance between two adjacent wheels 36 along the path so that two consecutive wheels 36 will simultaneously engage rigid traction surface 40 for a short time as the drive shoe advances from one wheel to another. However, in an accelerator or decelerator when simultaneous engagement occurs, slippage on surface 40 also must occur at one or the other wheel 36 because of the difference in angular velocities between two adjacent engaging wheels 36. Although wheels 36 are semi-pliable, such resiliency is inadequate to compensate for the relative rigidity of traction surface 40. Such resulting slippage promotes tire deflation, as well as accelerating tire wear and replacement. This problem of tire wear in accelerators and decelerators has been addressed by providing drive wheels that are mounted on spring biased axle assemblies which permit limited angular displacement during driving when adjacent drive wheels fight each other while simultaneously engaging the drive shoe. Such drive wheel mounting assemblies, however, do add to the overall cost of accelerators and decelerators.
Other examples of aerial tramway systems employing drive wheels to propel detached passenger carrier units are shown in U.S. Pat. Nos. 4,050,385; 3,685,457; 3,662,691 and 3,596,607; and German Patent No. 1,505,985; European Patent No. 0,125,967; French Patent Nos. 2,496,029 and 1,453,517; and German Patent Nos. 1,131,718 and 2,060,030. As is true of the general purpose transport systems, these aerial tramway systems are based upon frictional engagement between a drive assembly and a rigid drive or traction surface. In some instances, however, a rotatable drive member is carried by the transport unit but it is effectively fixed or rigid in its relationship to the chair, gondola or cabin.